Hi,
Unfortunately the only way to establish what part/s are damaged is to open up the iPad to visually assess the extent of water ingress to the internal parts (and after significant water damage you’ll be up for a new battery to start with).
As the iFixit guide for your iPad shows a very low repairability score, it’s probably not something you would do unless you have the experience or confidence & tools to look into it, you can also follow YouTube videos on how to treat water damage to iPads.
Alternatively you can take it to a repair shop and pay for an assessment & then decide if the cost is palatable.
Hope this helps